Natural gas is the most important fuel gas in the United States and provides more than one-fifth of all the primary energy used in the United States. Natural gas is also used extensively as a basic raw material in the petrochemical and other chemical process industries. The composition of natural gas varies widely from field to field. For example, a raw gas stream may contain as much as 95% methane, with only minor amounts of other hydrocarbons, nitrogen, carbon dioxide, hydrogen sulfide, or water vapor. On the other hand, streams that contain relatively large proportions of heavier hydrocarbons and/or other contaminants are common. Before the raw gas can be sent to the supply pipeline, it must usually be treated to remove at least one of these contaminants.
As it travels from the wellhead to the processing plant and, ultimately, to the supply pipeline, gas may pass through compressors or other field equipment. These units require power, and it is desirable to run them using gas engines fired by natural gas from the field. Since the gas has not yet been brought to specification, however, this practice may expose the engine to fuel that is of overly high Btu value, low methane number, or is corrosive.
In the gas processing plant itself, heavy hydrocarbons are often removed by condensation. Such a method is impractical in the field, however, because sources of external cooling or refrigeration are not available. Furthermore, cooling of the raw gas, which still contains substantial quantities of water vapor, is likely to bring the gas to a pressure/temperature/composition condition under which hydrates can begin to crystallize, thereby clogging the condensation equipment and preventing gas flow.
That membranes can separate C3+ hydrocarbons from gas mixtures, such as natural gas, is known, for example, from U.S. Pat. Nos. 4,857,078, 5,281,255; 5,501,722; and 6,053,965. Separation of acid gases from other gases is taught, for example, in U.S. Pat. No. 4,963,165. It has also been recognized that condensation and membrane separation may be combined, as is shown in U.S. Pat. Nos. 5,089,033; 5,199,962; 5,205,843; and 5,374,300.
Conventional membrane skids for use in fuel gas conditioning include at least two separate components: a filter element and one or more membrane vessels. Besides the separate vessels for the filters and membrane elements, the skid carries piping, valves, and other components needed to connect the filter and membrane vessels, as well as pipework and instrumentation to enable the filter/membrane skid to be tied in to the compressor skid or other equipment at the site. The costs for the piping and skid can be substantial, and the interconnecting piping may need to be insulated, heat-traced, and comply with specifications and codes to be used in the field. In addition, the skid itself requires longer to fabricate if piping is involved.
The traditional skidded approach described above is appropriate for larger fuel gas conditioning units (FGCUs) having relatively large numbers of membrane elements housed in multiple vessels. For smaller fuel gas conditioning applications that require only one or a few elements that can be housed in one membrane vessel, the relative costs of the pipework and frame become disproportionately high, and the time and complexity of installation discourage potential users due to price. There remains a need for simpler, more cost-effective equipment, especially where the gas to be conditioned has a relatively small flow rate.